The long term goals of my research are to understand the molecular mechanisms of specific cell-cell recognition and establish how these events at the cell surface lead to secondary responses by cells. l have chosen the mating reaction (fertilization) of the biflagellated alga, Chlamydomonas reinhardtii, as a model system for these studies. Fertilization is initiated in this organism when gametes of opposite mating types adhere to each other via agglutinin molecules on their flagellar surfaces. Agglutinin interactions activate a flagellar membrane adenylyl cyclase, resulting in increased levels of intracellular cAMP and thereby initiating a complex cellular response that prepares the interacting gametes for cell fusion. In our recent studies to elucidate the molecular mechanisms of signal transduction in this system we have discovered what we believe is a complex, multistep pathway requiring protein phosphorylation and dephosphorylation that couples agglutinin interactions to activation of adenylyl cyclase. We have found that a protein kinase is required to activate adenylyl cyclase in adhering gametes, that a protein kinase is inactivated during flagellar adhesion, and finally, we have purified, cloned and sequenced a new soluble flagellar protein that is a substrate for this adhesion-regulated protein kinase and discovered that the substrate itself is a novel protein kinase. The research described in this application is intended to accomplish the following: 1) Identify and characterize the adhesion-regulated protein kinase that phosphorylates the soluble flagellar protein. 2) Study regulation of the new soluble protein kinase/protein kinase substrate and determine its physiological role in adhesion-induced signal transduction. 3) Identify and characterize the adenylyl cyclase. 4) Use molecular methods to characterize flagellar agglutinin. These studies should provide new information about molecular mechanisms of cell-cell interactions, which are important in the proper functioning of the immune system, in metastasis, and in fertilization and early development.